Photoelectric device and method of manufacture



PHOTOELECTRI C DEVICE AND METHOD OF MANUFACTURE Filed Feb. 27, 1940 INVENTOR. WILLARD HICKOK ATTORNEY.

Patented Sept. 16, 1941 PHOTOELECTBJC DEVICE AND METHOD OF MANUFACTURE Willard Hickok, Bloomfield, N. L, asslgnor to Radio Corporation of America, a Delaware corporation Application February 2'7, 1940, Serial No..320,995

11 Claims.

My invention relates to photo-electric tubes, photocathode structures for use in such tubes and a method of making such structures.

It is an object of my inventionto provide a photo-electronically emissive electrode having higher photosensitivity than such electrodes of the conventional type. It is another object to provide a more satisfactory method of manufacture for obtaining the desired spectral or color response of a p'hotocathode. It is a further .object to provide an electrode and a method of manufacturing such an electrode having decreased surface reflection and consequent higher light absorption, and it is a still further object of my invention to provide a mosaic electrode having low secondary electron emission when scanned by a high velocity electron beam and consequent increased ratio of signal output to spurious signal, while at the same time providing an electrode capable of producing an increased signal output.

In accordance with my invention I make a metallic foundation which may be of either the continuous or of the discontinuous mosaic type 7 so that the portion which is subsequently rendered photosensitive is porous, whereby the surface area of the structure is increased while at the same time the photo-electric emission is controlled by reason of the porosity of the foundation. My invention is more than a mere roughening of the foundation to increase the surface, as the foundation itself is porous in such a way that voids exist both on the surface of the foundation and within the material comprising the foundation for the photo-electric cathode. These and other objects, features and advantages of my invention will appear when taken in connection with the accompanying drawing in which:

Figure 1 shows a tube of the television transmitting type incorporating an electrode structure made in accordance with my invention,

Figures 2a, 2b and 2c are sectional views of an electrode structure of the mosaic type following three consecutive steps in my new and improved method of manufacture,

Figures 3 and 4 are microphotographs, and Figure 40 an enlargement of a portion of Figure 4, showing a mosaic prior to and after certain steps in my manufacturing process, and

Figure 5 is a view of a phototube incorporating a cathode of my new and improved type.

Referring to Figure 1 which shows a television tube having a photocathode or mosaic electrode made in accordance with my invention, the tube comprises a highly evacuated glass envelope or bulb l with a tubular arm or neck section enclosing a. conventional type electron gun and a spherical section enclosing a. flat target or mosaic electrode 2 so positioned that its front surface may be scanned by a beam of electrons from the electron gun and also may have projected upon it the optical image to be transmitted. Since the image is produced from an object situated outside the tube, that portion or window 3 of the spherical section opposite the electrode 2 is made optically uniform so that the image to be transmitted may be projected upon the electrode with a minimum of distortion by the lens system 4.

The electron gun assembly is of the conven tional type, and comprises a cathode 5 from which an electron stream may be drawn, a control electrode 6 connected to the usual biasing battery, and a first anode I maintained positive with respect to the cathode 5 by a battery 8. The electron stream leaving the first anode 1 is accelerated and concentrated into an electron scanning beam focused on'the front surface of the target 2 by a second anode 9, which is preferably a conductive coating on the surface of the envelope I near the neck of the bulb but removed from that portion through which is projected the optical image to be transmitted. Conventional deflection means, such as deflection coils I0 and II may be used to sweep the beam in a horizontal and vertical plane, respectively, to scan the target. It is obvious that conventional electrostatic deflection plates may be substituted for one or both of the deflection coils if desired. The electrode 2 is connected through the impedance l2 to ground and to the collector electrode or second anode 9, and in operation the current flow in this circuit produces a voltage drop across the impedance l2 which may be impressed on the inputof a translating device I3, further amplified and applied to a transmitting network in a manner well known to the art.

As best shown in Figure 2a, an insulating foundation sheet or'base such as a sheet of mica is coated with metal or other electrically conducting material, such as a film of platinum or finely divided carbon which is co-extensive with the base and serves as a signal electrode from which the picture signals may be obtained. The other side of the sheet of mica has on its surface a multiplicity of minute, individually separated particles of metal such as silver which serve as foundation elements for the photosensitive material subsequently deposited. These elements, in capacitive relationship with the signal electrode, liberate electrons and acquire an electrostatic charge when illuminated,and when scanned by an electron beam the elements become discharged, thereby generating a signal through the translating device It. In accordance with my invention I treat the mosaic particles in such a manner that they become porous and expand so that the enclosed volume of the particles is greater than before treatment. More particularly, I choose -a material for the particles which has the property of readily amalgamating with mercury or mercury vapor to which the particles may be subjected. I have previously described in my U. S. Patent 2,178,232 a method of forming particles of silver on a nonconducting base so that this portion of the structure will not be repeated in detail in the following description of my invention.

The mosaic electrode 2 includes an insulating foundation or base such as a sheet of mica l4, Figure 2a, provided on one exposed surface with a film of platinum It or other conducting material. The opposite surface of the sheet of mica It carries the individual and minute portions of silver particles l6 spaced from each other, the number and average size of particles in a unit area being sufficient to satisfy the operating requirements in the way of detail of picture reproduction. 'The particles of silver may be formed from settled particles of silver oxide or by vaporizing a film of silver, depositing it on the mica and breaking it up into individual particles as disclosed by S. F. Essig in U. S. Patent 2,065,570. Following the formation of the silver particles, I subject the particles and the sheet of mica to mercury vapor, maintaining the sheet of mica relatively cool so that mercury is deposited on the silver particles to form a silver amalgam. Because of the small size of the particles the mercury permeates the particles very readily and causes the particles It to expand and occupy a somewhat larger volume, as shown at that necessary to form a solid amalgam to prevent the particles from spreading too much. It is also preferable to retain a base of solid silver in contact with the mica sheet so that the particles remain in good contact with the mica. In

-carrying out the mercury treating step of my invention I- provide a beaker having a cross-section corresponding with the outline of the sheet of mica It so that the mica may be supported over the opening in the beaker. I place a few drops of mercury in the beaker and heat the mercury so that a portion thereof is vaporized. This step may be performed upon an ordinary hot plate in a ventilated hood, the sheet of mica being cooled either by contact with the surrounding atmosphere or by a blast of air at room temperature directed upon the surface of the sheet opposite that carrying the silver particles subjected to the mercury vapor. Prior to the above step the particles, when viewed through a microscope, appear to be highly reflective to light, have a smooth surface and are substantially hemispherical, with the base portion in contact with the mica sheet, as shown in Figure 2a. Following the treatment with mercury, the particles ltb of silver with the amalgamating mercury are considerably duller and have spread laterally to cover a somewhat larger area on the mica sheet, as shown in Figure 2b. Following. the amalgamation of the silver particles, the mica sheet is baked at a temperature suflicient to evaporate the mercury from the silver particles, leaving a silver residue of the amalgam, this baking preferably being done rapidly so that the mercury is quickly volatilized. When the mercury is volatllized, small voids or pockets are formed in the silver connected with the surface by small fissures, rendering the surface very rough and nonreflective to light. I have found that it is preferable to insert the mica sheet in a pre-heated furnace at a temperature of 600 C. Since little or no oxidation of the silver particles occurs, this flring may be done in air. The time of firing is immaterial because after approximately 15 seconds in the pre-heated furnace substantially all of the mercury has been evaporated, although to be certain that all mercury has been volatilized, I prefer to bake the sheet of mica for'l minute or longer. The use of lower firing temperatures naturally increases the time of firing. An examination of the silver particles will reveal that each particle has a dull finish and is pitted, these pits or fissures extending into the interior of the silver particles; and in addition, the particles are rendered porous, that is, voids exist within the particles themselves, as shown by lSc, Figure 2c. It will be observed from the microphotograph, Figure 3, which is a portion of a mosaic surface prior to the mercury treatment magnified 1000 diameters, that the silver particles appear as white spots. This is due to their high reflectivity of light because the particles are relatively smooth and have an unbroken surface. When such a smooth surface is scanned by an electron beam, the ratio of secondary electrons emitted by the surface to'incident electrons is quite high, o'ften exceeding 10 to 1. These secondary electrons produce spurious signals which are very difllcult to compensate. Referring to Figure 4, however, which shows a similar microphotograph subsequent .to the mercury treatment and baking steps and to Figure 4a which shows greatly enlarged a single particle appearing in Figure 4, it is apparent that the silver particles have been rendered porous and that the surface of the particles is crazed and flssured and is relatively rough, a condition conducive to low secondary electron emission, while at the same time reducing the reflection of light from the particles. Since the reflection of the incident light is reduced, more light is available for producing electronic emission and a higher photosensitivityis obtained with such a roughened surface.

Following the baking step, the sheet of mica is ready to be assembled on a supporting framework and mounted in the tube. Following the mounting of the mica sheet within the envelope I of Figure l, the silver particles are oxidized by subjecting the particles to a glow discharge in oxygen and photosensitized by exposure to caesium vapor in the usual way. Somewhat more caesium may be used than in sensitizing the conventional mosaic because of the additional surface produced by the flssural and porous character of the silver particles. Following the exposure of the particles to caesium vapor, the tube is baked and simultaneously pumped until maximum photosensitivity is obtained and any free caesium deposited on the mica or on a film of cryolite intermediate the particles and the mica as described in my above-mentioned patent is vaporized to reduce surface leakage between the photosensitized silver particles.

While I have described my invention in connection with a photosensitive structure of the mosaic type, my invention is nevertheless applicable to photosensitive cathodes of the continuous type such as used in the photo-electric tube shown in Figure 5. The cathode 20 of the tube shown in Figure is preferably of relatively thick silver, althoughit may be thinner if supported by a relatively heavy base 2| of nickel, copper or other suitable metal. The nickel or copper plate with its coating of silver or a solid silver plate to serve as the cathode 20 is exposed to mercury vapor which is allowed to condense thereon, forming an amalgam with the silver. The amount of mercury condensed on the silver is not critical, in fact, it may be sumcientto run off as a liquid. Alternatively the cathode 20,

may be dipped in mercury, the excess which; clings to the silver being removed by agitation. Following the deposition of mercury on the silver surface, the cathode 20 is rapidly heated as indicated above to drive off the mercury, leaving a flssural matte electrode having a crazed surface. Under microscopic examination the surface is seen to be flssured or pitted and the underlting metal to be porous. The metal under the porous layer may be solid and integral with the porous layer whose thickness depends on the penetration of the mercury into the thickness of the cathode. Following the vaporization of the mercury from the cathode foundation, the foundation may be sealed within an envelope 22 as shown in Figure 5 in cooperation with an anode 23, the envelope having a press carrying lead wires 24 individually connected with the cathode 20 and anode 23. The photosensitization of the cathode with caesium or other alkali. metal may be by conventional processes, although I have found the following process to be particularly suitable in obtaining a phototube having high photosensitivity over the visible portion of the spectrum.

The envelope 22 is exhausted by sealing it to an exhaust manifold and evacuating it through an exhaust tubulation 25. The tube is baked at a temperature of 390-410 C. until a residual pressure of 1 micron or less is reached, following which the baking and exhaust are continued for at least 5 minutes. The temperature of the tube is then reduced to that of the surrounding atmosphere (room temperature) and oxygen is admitted to a pressure of 1.2 mm. Hg. The cathode 20 is then oxidized by subjecting it to a glow discharge until the cathode is oxidized to the first yellow. Additional oxygen is admitted to the envelope to a pressure of 1.8 mm. Hg and the cathode 20 is further oxidized to a bright green following a bright red. The residual oxygen is then removed and a source of caesium or other alkali metal 26, retained on the metal tab 21, is heated to vaporize the alkali metal. The amount of metal such as caesium depends upon the area of the oxidized surface of the cathode and since this surface is porous in charthe mosaic type is particularly useful in the manufacture of electrodes where the electrode is to be scanned by an electron beam. Since con ventional light sources used in television transmission are high in the red portion of the spectrum and photo-electrons liberated in response to this portion have relatively low velocities of.

- in higher eflective photosensitivity in the higher acter, somewhat more caesium should be used I than in conventional phototubes. Some slight excess may be used which may be removed by baking at a temperature of 265-285" C. until the cathode turns to a gold or straw color, whereupon the exhaust tubulation 25 may be sealed ofl.

My method of processing photocathodes of wave length portion of the spectrum. In addition, secondary electrons produced by an electron beam scanning the electrode are eflectively trapped by the porous nature of the electrode, thereby reducing the distribution of secondary electrons over adjacent areas of, the electrode. In conventional phototubes such as shown in Figure 5 the porous character of the emissive surface limits the red photosensitivity of the tube, while at the same time, provides additional surface over and above that which may be obtained from an etched or mechanically roughened surface, thereby resulting in a higher over-all sensitivity of the photocathode structure.

Alternative methods of making a mosaic electrode do not result in obtaining the desired crazed or fissured surface. It is practically impossible to etch the minute particles of a mosaic because during the etching much of the metal of the particles would be removed and many of the particles would be destroyed, since the particles are of non-uniform size and are distributed at random over the sheet of mica. It is obvious that electrolytic deposition 'of the mosaic particles is impractical because the particles must be deposited on an electrical insulator and there is no metallic or conductive foundation to serve as the cathode in an electrolytic process. While the electrolytic deposition process is suitable for making some types of continuous surface photocathodes, the resulting surface with high current density electrodeposition has a mossy appearance which is not crazed or flssured. In addition, the surface of an electrode made by high current density deposition is very fragile, any mechanical abrasion after the deposition causing the mossy surface to be rubbed ofi. In making my new and improved electrode the mercury treatment does not destroy the bond between the treated metal and the foundation metal, and. consequently, the'electrode is very rugged and may be handled without fear of destroying the crazed or flssured porous structure.

While I have indicated the preferred embodiments of my invention and have also indicated two specific applications for which my invention may be employed, it will be apparent that my invention is by no means limited tothe exact forms and method of manufacture disclosed or the use indicated, but that many variations may be made in the materials and mode of manufacture, such as by depositing mercury onthe foundation by electrolytic processes, and the purpose for which it is employed without departing from the scope of my invention as set forth in the appended claims.

I claim:

1. The process of fabricating a photosensitive electrode which comprises the steps of subjecting to mercury a metal capable of forming an amalgam, heating the amalgamated metal to drive oif substantially all of the mercury and then photosensitizing the metal.

2. The process of fabricating a photosensitive electrode which comprises the steps of forming an amalgam on a metal which, when subjected to the vapor of an alkali metal, becomes photoelectronically emissive, heating the amalgam to a temperature sufficient to vaporize the mercury from the amalgam, and photosensitizing the metal by subjecting it to the vapor of an alkali metal.

3. The process of fabricating a photosensitive electrode which comprises the steps of forming a base of silver, subjecting the silver to mercury to form an amalgam, heating the amalgamated base to remove substantially all of the mercury from the amalgam, and then photosensitizing the treated silver base.

4. The process of fabricating a photosensitive electrode which comprises subjecting a silver base to mercury vapor to form an amalgam, vaporizing substantially all of said mercury from said amalgam to form a fissured porous silver base and photosensitizing said base by subjecting it to the vapor of an alkali metal.

- 5. The process of forming a porous fissured silver base for a photosensitive electrode which includes the steps of subjecting a silver base to mercury vapor, to deposit mercury vapor thereon, and rapidly heating said base to vaporize and remove substantially all of said mercury from said base.

6. The process of forming a porous flssured silver electrode for a photosensitive device which comprises the steps of coating a silver base with mercury to form an amalgam, rapidly heating the amalgamated silver base to vaporize the mercury from the amalgam rapidly, oxidizing the base, and subjecting the oxidized base to the vapor of an alkali metal to photosensitize said base.

7. The process of fabricating a mosaic electrode comprising the steps of forming a multiplicity of mutually separated metallic particles of a metal which amalgamates with mercury on an insulating base, depositing mercuryon said particles to form an amalgam, and heating the particles to vaporize substantially all of the mercury from said particles.

8. The process of fabricating a mosaic electrode comprising the steps of forming a multiplicity of closely adjacent mutually separated silver particles on an electrically insulating base, subjecting the particles to mercury vapor to deposit mercury thereon, limiting the deposition of mercury on said particles to an amount insumcient to form a liquid amalgam andcause the particles to spread into contact with one another, and vaporizing the mercury deposited on said particles.

9. The process claimed in claim 8 including the additional steps of oxidizing the silver particles after vaporizing the mercury therefrom and subjecting said particles to the vapor or an alkali metal to photosensitize said particles.

10. The process of fabricating a mosaic electrode comprising the steps of forming a multiplicity of silver particles on an electrically insulatlng base, forming a solid amalgam with said particles and rapidly heating said base and said particles to volatilize the mercury forming the solid amalgam.

11. A photosensitive electrode of the mosaic type comprising an electrically non-conducting base and a multiplicity of particles comprising the silver residue of amalgamated silver and containing substantially no mercury.

WILLARD HICKOK. 

